r^ 



482 



Organs of Vision), and this gradually results in the change 

 in shape of the whole optic region, till eventually in the three- 

 day pupa the optic ganglion is mainly in the form of a short 

 stalk which connects the eye with the rest of the bra^in, while 

 the other two optic ganglia lie in a projection of the hemi- 

 spheres rather narrower than that in which they lay in the 

 adult larva (figs. 80, 234, 236). 



In the ^arly pupa, too, the antennal ganglia grow largely 

 in size (fig. 232), and form a pair of very distinct antennal 

 lobes projecting downwards, forwards, and slightly inwards 

 from the antero- ventral part of each hemisphere. The ocellar 

 anglia also project slightly on the surface of the brain. 



In the last hours of larval life a development of the 

 nerve fibres has commenced. This is rather difficult to observe, 

 for the newly formed nerve fibres grow into the degenerate 

 mass of old nerve fibres, absorbing them as they grow, and as 

 the latter disappear, the others replace them, there being no 

 visible discontinuity. The only visible sign of change is 

 a gradual resumption of fibrillated appearance by the 

 degenerate masses of nerve strands. The new ne^'ve strands 

 thus formed are best studied in the optic region. Between the 

 outer and middle optic ganglia such a strand, rather short 

 and thickset, and never very prominent, gradually appears, 

 quite independently, in this case, of the old larval nerve 

 strand. It corresponds to the -perio'pticon of Hickson (figs. 80, 

 236), and is formed by fibres some of which have grown 

 inwards from the outer ganglion, others outwards from the 

 middle ganglion. Synapses are doubtless formed between the 

 two. 



The norve cells comprising the middle and inner optic 

 ganglia likewise develop fibres, which grow, this time, 

 through the old larval nerve strand. They evidently form 

 synapses here, and the whole structure forms the second mass 

 of nerve fibres, very well developed in the imago, and con- 

 stituting the "epiopticon" of Hickson (fig. 236). The nerve 

 cells of the inner ganglion likewise give off processes inwards 

 along the old larval nerve strand, and they and similar fibres 

 from more internal parts of the brain unite to form the 

 "opticon" of Hickson — the third optic nerve strand, which 

 finally brings the optic nerves into communication with the 

 rest of the brain. These changes take place in the early pupa, 

 and so far as it is possible to observe them, are complete at 

 the end of about the first day of pupal life. Many of the 

 cells thus produced are true bipolar nerve cells, but many of 

 the fibres which help to form these large nerve strands come 

 from masses of cells which have not grown into the brain, 

 but have remained more at the periphery. Although I have 



